1. Technical Field
The invention relates to image processing. More particularly, the invention relates to the descreening of halftoned images.
2. Description of the Prior Art
Halftone techniques have long been used to produce color images using a small number of inks and a restricted set of densities (typically 100%/0%, or ink/no ink). In digital imaging halftoning usually involves a threshold array, which is tiled to cover the whole page, or a dynamic error-diffusion procedure that averages the errors due to the device limits throughout the page.
When such images (that were originally printed using a halftone technique, either digital or analog), are digitally captured by a scanning device, it is often desirable to restore the original color intensities (or ink densities) and obtain a contone equivalent of the halftoned image. This process is often referred to as descreening because it removes the screen that was applied while halftoning. Failure to remove the low frequencies that were introduced through the halftoning results in artifacts when the scanned image is either displayed or reproduced in print.
Various descreening approaches have been described in the art. See, for example, J. Stoffel, Half Tome Encoder/Decoder, U.S. Pat. No. 4,193,096 (Mar. 11, 1980); P. Roetling, Unscreening of Stored Digital Halftone Images, U.S. Pat. No. 4,630,125 (Dec. 16, 1986); H.-T. Tai, Image Processing Method To Remove Halftone Screens, U.S. Pat. No. 5,239,390 (Aug. 24, 1993); D. Seidner, D. Eylon, Apparatus and Method For Descreening, U.S. Pat. No. 5,384,648 (Jan. 24, 1995); P. Lavelle, J. Stoffel, Multi-Resolution Image Signal Processing Apparatus and Method, European Patent No. 041400 (Jun. 6, 1984); R. Eschbach, Image-Dependent Exposure Enhancement, European Patent No 648040 (Nov. 2, 1995); Z. Xie, M. Rodriguez, Electronic High-Fidelity Screenless Conversion System and Method Using A Separate Filter, European Patent No. 581415 (Feb. 23, 1994); and P. Stansfield, A. Reed, Image Processing, European Patent Application No. 301786 (Feb. 1, 1989).
Unfortunately, such prior art techniques are either concerned with such issues as compression/decompression or descreening only in circumstances where the actual screen pattern is known prior to descreening.
It would be advantageous to provide an improved descreening technique. It would be further advantageous to provide an improved technique for descreening halftone images, where the halftone screen pattern is not known prior to descreening.